Dynamic covalent chemistry of a boronylammonium ion and a crown ether: formation of a C3-symmetric [4]rotaxane

Structural Interconversion Based on Intramolecular Boroxine Formation

A novel structural interconversion unit based on intramolecular boroxine formation has been developed. A macrocyclic triboronic acid consisting of three phenylboronic acid units linked by covalent linkers preferentially underwent intramolecular rather than intermolecular boroxine formation, resulting in a quantitative formation of tricyclic boroxine. This structural transformation was accompanied by changes in the polarity, flexibility, and size of the molecule. Dynamic interconversion between the macrocyclic triboronic acid and the tricyclic boroxine was achieved by simple heating/cooling, whereas no boroxine formation occurred upon heating when three boronic acid units were not connected by linkers. Thermodynamic analysis revealed that the entropic advantage of the intramolecular boroxine formation process resulted in these unique features. The entropically stabilized tricyclic boroxine also shows high stability with respect to hydrolysis.

Discrete boronate ester ladders from the dynamic covalent self-assembly of oligo(phenylene ethynylene) derivatives and phenylenebis(boronic acid)

Reversible boronate ester chemistry enables the controlled, dynamic self-assembly of olig(phenylene ethynylene)s into highly conjugated ladder frameworks.

Formation of [2]- and [3]Rotaxanes through Bridging under Kinetic and Thermodynamic Control

An efficient synthesis of a doubly stranded [3]rotaxane has been developed through bridging of a pseudo[3]rotaxane featuring two axle components. Reversible azine formation was effective as the bridging reaction. Kinetic and thermodynamic conditions provided the [2]- and [3]rotaxanes, respectively.

Unveiling a New Aspect of Simple Arylboronic Esters: Long-Lived Room-Temperature Phosphorescence from Heavy-Atom-Free Molecules

Arylboronic esters can be used as versatile reagents in organic synthesis, as represented by Suzuki-Miyaura cross-coupling. Here we report a serendipitous finding that simple arylboronic esters are phosphorescent in the solid state at room temperature with a lifetime on the order of several seconds. The phosphorescence properties of arylboronic esters are remarkable in light of the general notion that phosphorescent organic molecules require heavy atoms and/or carbonyl groups for the efficient generation of a triplet excited state. Theoretical calculations on phenylboronic acid pinacol ester indicated that this molecule undergoes an out-of-plane distortion at the (pinacol)B-C_ipso moiety in the T₁ excited state, which is responsible for its phosphorescence. A compound survey with 19 arylboron compounds suggested that the phosphorescence properties might be determined by solid-state molecular packing rather than by the patterns and numbers of boron substituents on the aryl units. The present finding may update the general notion of phosphorescent organic molecules.

Reversible covalent chemistries compatible with the principles of constitutional dynamic chemistry: new reactions to create more diversity

An approach to make chemical diversity space more manageable is to search for smaller molecules, or fragments, and then combine or elaborate these fragments. Dynamic Combinatorial Chemistry (DCC) is a powerful approach whereby a number of molecular elements each with binding potential can be reversibly combined via covalent or noncovalent linkages to generate a dynamic library of products under thermodynamic equilibrium. Once a target molecule has been added, the distribution of products can be shifted to favor products that bind to the target. Thus the approach can be employed to identify products that selectively recognize the target. Although the size of the repertoire of reversible covalent reactions suitable for DCC has increased significantly over the past 5-10 years, the discovery of new reactions that satisfy all the criteria of reversibility and biocompatibility remains an exciting challenge for chemists. Increasing the number of chemical reactions will enable the engineering of larger and more diverse DCLs, which remains a key step toward a broader use of DCC. In this review, we aim to provide a nonexhaustive list of reversible covalent reactions that are compatible with the concept of DCC, focusing mainly on the most recent examples that were reported in the literature in the past 5 years.

Recent advances in the synthesis of ammonium-based rotaxanes

The number of synthetic methods enabling the preparation of ammonium-based rotaxanes has increased very rapidly in the past ten years. The challenge in the synthesis of rotaxanes results from the rather weak interactions between the ammonium-containing rod and the crown ether macrocycle in the pseudorotaxane structure that rely mostly on O*H hydrogen bonds. Indeed, no strong base or polar solvent that could break up H-bonding can be used during the formation of rotaxanes because the two components will separate as two distinct entities. Moreover, most of the reactions have to be performed at room temperature to favor the formation of pseudorotaxane in solution. These non-trivial prerequisites have been taken into account to develop efficient reaction conditions for the preparation of rotaxanes and those are described in detail along this review.